shell: fix MISRA 5.7 violations on `struct shell`

MISRA Rule 5.7 requires uniqueness of tag identifiers. Shell is
frequently problematic because many code uses `const struct shell
*shell`. This causes CI noise every time one of these shell files is
edited, so let's update all of them with `const struct shell *sh`
instead.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>